1. introduction to electrical drives.pdf
TRANSCRIPT
Introduction of Electrical DrivesByH. S. DarjiDepartment of Electrical EngineeringU. V. Patel College of Engineering
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Reference Books
Definition of Electrical Drives
“An electrical drive is defined as a form of machineequipment designed to convert electrical energy intomechanical energy & provide electrical control of thisprocess.”
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Block diagram of an Electric Drives
Small (compact)
Efficient
Flexible
Interdisciplinary
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Power SourcePower Processing
UnitMotor Load
Control
Reference
Control
Unit
feedback
Basic Components of Electric Drives
Power Source
Motor
Power Processing Unit (Electronic Converter)
Control Unit
Mechanical Load
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Power SourcePower
Processing Unit
Motor Load
Control Reference
Control
Unit
feedback
Basic Components of Electric Drives – Power Source• Provides energy to electric motors• Regulated (e.g: utility) or Unregulated (e.g. : renewable
energy)• Unregulated power sources must be regulated for high
efficiency – use power electronic converters• DC source
• batteries• fuel cell • photovoltaic
• AC source• single- or three- phase utility• wind generator
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Basic Components of Electric Drives - Motor
• Obtain power from electrical sources
• DC motors - Permanent Magnet or wound-field (shunt, separately excited, compound, series)
• AC motors – Induction, Synchronous (wound –rotor, IPMSM, SPMSM), brushless DC
• Selection of machines depends on many factors, e.g.:
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Electrical energy
Mechanicalenergy
Motor
• application• cost• efficiency
• environment • type of source available
Basic Components of Electric Drives – Power Processing Unit• Provides a regulated power supply to motor• Enables motor operation in reverse, braking and variable
speeds• Combination of power electronic converters Controlled rectifiers, inverters –treated as ‘black boxes’
with certain transfer functionMore efficient – ideally no losses occurFlexible - voltage and current easily shaped through
switching controlCompact Several conversions possible: AC-DC , DC-DC, DC-AC, AC-AC
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Basic Components of Electric Drives – Power Processing Unit DC to AC:
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Basic Components of Electric Drives – Power Processing Unit DC to DC:
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Basic Components of Electric Drives – Power Processing Unit AC to DC:
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Basic Components of Electric Drives – Power Processing Unit AC to AC:
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Basic Components of Electric Drives – Control Unit• Supervise operation
• Enhance overall performance and stability
• Complexity depends on performance requirement
• Analog Control – noisy, inflexible, ideally infinite bandwidth
• Digital Control – immune to noise, configurable, smaller bandwidth (depends on sampling frequency)
• DSP/microprocessor – flexible, lower bandwidth, real-time
• DSPs perform faster operation than microprocessors (multiplication in single cycle), complex estimations and observers easily implemented
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Advantages of Electrical Drives Flexible control characteristic particularly when power electronic converters are
employed
Wide range of speed, torque and power
High efficiency – low no load losses
Low noise
Low maintenance requirements, cleaner operation
Electric energy easily transported
Adaptable to most operating conditions
Available operation in all four torque-speed quadrants
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Choice of Electrical Drives• Several factors affecting drive selection:
• Steady-state operation requirements• nature of torque-speed profile, speed regulation, speed range, efficiency,
quadrants of operations, converter ratings
• Transient operation requirements• values of acceleration and deceleration, starting, braking and reversing
performance
• Power source requirements• Type, capacity, voltage magnitude, voltage fluctuations, power factor,
harmonics and its effect on loads, ability to accept regenerated power
• Capital & running costs
• Space and weight restrictions
• Environment and location
• Efficiency and reliability
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Electric Drives Application
Line Shaft Drives
Oldest form
Single motor, multiple loads
Common line shaft or belt
Inflexible
Inefficient
Rarely used
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Electric Drives Application Single-Motor,
Single-Load Drives
Most common
Eg: electric saws, drills, fans, washers, blenders, disk-drives, electric cars.
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Electric Drives Application Multimotor Drives
Several motors, single mechanical load
Complex drive functions
Eg: assembly lines, robotics, military airplane actuation.
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DC or AC Drives?DC Drives
AC Drives (particularly Induction Motor)
Motor • requires maintenance• heavy, expensive• limited speed (due to
mechanical construction)
• less maintenance• light, cheaper• high speeds achievable (squirrel-
cage IM)• robust
Control Unit Simple & cheap control even for high performance drives• decoupled torque and flux
control• Possible implementation using
single analog circuit
Depends on required drive performance• complexity & costs increase with
performance• DSPs or fast processors required in
high performance drives
Performance Fast torque and flux control Scalar control – satisfactory in some applicationsVector control – similar to DC drives
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Torque Equation for Rotating Systems Motor drives a load through a transmission system (eg.
gears, V-belts, crankshaft and pulleys)
Load may rotate or undergo translational motion
Load speed may be different from motor speed
Can also have multiple loads each having different speeds, some may rotate and some have translational motion
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Motor Load
Te , m TLRepresent motor-
load system as equivalent
rotational system
Torque Equation for Rotating Systems
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• First order differential equation for angular frequency (or velocity)• Second order differential equation for angle (or position)
2
2
dt
dJ
dt
dJTT m
Le
With constant inertia J,
dt
JdTT m
Le
Te , m
TL
Torque equation for equivalent motor-load system:
where:J = inertia of equivalent motor-load system, kgm2
m = angular velocity of motor shaft, rads-1
Te = motor torque, NmTL = load torque referred to motor shaft, Nm
(1)
(2)
Torque Equation for Rotating Systems with Gears Low speed
applications use gears to utilize high speed motors
Motor drives two loads: Load 1 coupled
directly to motor shaft
Load 2 coupled via gear with n and n1teeth
Need to obtain equivalent motor-load system
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Motor
Te
Load 1,
TL0
Load 2,
TL1
J0
J1
mm
m1
n
n1
TL0
TL1
Motor
Te
J
Equivalent
Load , TL
m
TL
Torque Equation for Rotating Systems with Gears Gear ratio a1 =
Neglecting losses in the transmission:
Hence, equivalent motor-load inertia J is:
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Kinetic energy due to equivalent inertia
= kinetic energy of moving parts
1
2
10 JaJJ
(3)
(4)
Torque Equation for Rotating Systems with Gears If 1 = transmission efficiency of the gears:
Hence, equivalent load torque TL is:
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Power of the equivalentmotor-load system
= power at the loads
1
110
L
LL
TaTT (5)
Torque Equation for Rotating Systems with Translational Motion Motor drives two
loads: Load 1 coupled
directly to motor shaft
Load 2 coupled via transmission system converting rotational to linear motion
Need to obtain equivalent motor-load system
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Motor
Te
J
Equivalent
Load , TL
m
TL
Torque Equation for Rotating Systems with Translational Motion Neglecting losses in the transmission:
Hence, equivalent motor-load inertia J is:
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Kinetic energy due to equivalent inertia
= kinetic energy of moving parts
2
110
m
vMJJ
(7)
Torque Equation for Rotating Systems with Translational Motion If 1 = transmission efficiency of the transmission system:
Hence, equivalent load torque TL is:
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Power of the equivalentmotor-load system
= power at the loads and motor
m
LL
vFTT
1
1
10
(8)
Torque Equation for Rotating Systems – Example
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Components of Load Torque(Tl)• Load torque can be divided into:
• Friction torque (TF) -present at motor shaft and in various parts of load.
• Viscous friction torque Tv – varies linearly with speed (Tv m). Exists in lubricated bearings due to laminar flow of lubricant
• Coulomb friction torque TC – independent of speed. Exists in bearings, gears coupling and brakes.
• Windage torque (Tw)-exists due to turbulent flow of air or liquid.
• Varies proportional to speed squared (Tw m2).
• Mechanical Load Torque (TL ) - torque required to do the useful mechanical work.
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Mechanical Load Torque• Torque to do useful mechanical work TL – depends
on application.
• Load torque is function of speed
• where k = integer or fraction
• Mechanical power of load:
• and
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k
mLT
mLTP mm n60
2
Angular speed in rad/s
Speedin rpm
Torque-Speed Characteristics of Load
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1) Torque independent of speed
2) Linear rising Torque-Speed
3) Non-Linear rising Torque-Speed
4) Non-Linear falling Torque-Speed
Torque-Speed Characteristics of LoadTorque
independent of speed , k = 0
Hoist
Elevator
Pumping of water or gas against constant pressure
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Torque-Speed Characteristics of LoadTorque
proportional to square of speed , k= 2
Fans
Centrifugal pumps
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Torque-Speed Characteristics of LoadTorque inversely
proportional to speed , k = -1
Milling machines
Electric drill
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Classification of Electrical Drives Group Drive(Shaft Drive)
Individual Drive
Multi-Motor Drive
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Classification of Electrical DrivesGroup Drive(Shaft Drive)“If Several groups of Mechanisms or Machines are organized on
one shaft & driven by one motor, the system is called a groupdrive (Shaft Drive)”
Disadvantages There is no flexibility, Addition of an extra machine to the main
shaft is difficult. The efficiency of the drive is low, because of the losses occurring
in several transmitting mechanisms. The complete drive system requires shutdown if the motor,
requires servicing or repair. The system is not very safe to operate The noise level at the work spot is very high.
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Classification of Electrical DrivesIndividual Drive“If a single motor is used to drive a given mechanism & it
does all the jobs connected with load, the drive iscalled an individual drive”
Examples
• Single Spindle drilling machine
• Lathe machines
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Classification of Electrical DrivesMulti-Motor Drive“In a Multi-Motor drive, each operation of the
mechanism is taken care of by a separate drive motor.The system contains several individual drives, each ofwhich is used to operate its own mechanism”
Examples
• Metal cutting machine tool
• Rolling mills
• Travelling cranes
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Dynamic Conditions of a drive system
• Dynamic conditions occur in a electric drive systemwhen operating point changes from one steady statecondition to another, following a change introduced inthe system variables. This variables may bemechanical such as speed, torque etc. or electricalsuch as voltage, current etc.
• These conditions generally exist during starting,braking and speed reversal of the drive.
• The dynamic conditions arise in a variable speed drivewhen transition from one speed to another is required.
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Dynamic Conditions of a drive system
• The drive may also have transient behavior if thereare sudden changes of load, supply, voltage orfrequency.
• The dynamic behavior of a drive has a closerelation to its stability. A drive is said to be stable ifit can go from one state of equilibrium to anotherfollowing a disturbance in one of the parameters ofthe system.
• Stability can be identified as either steady-state ortransient.
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Dynamic Conditions of a drive system
• The condition of stability depend on the operatingpoint.
The dynamics of the drive can be investigated usingthe Torque balance equation given by
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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Dynamic Conditions of a drive system
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The load torque occurring in mechanical systemmay be Passive or active.
Passive torque
If the torque always opposes the direction of motionof drive motor it is called a passive torque.
Active torque
Load torque which have the potential to drive themotor under equilibrium condition are calledactive load torque.
Motor T- characteristic – variation of motor torque with speed with all other variables (voltage and frequency) kept constant.
Loads will have their own T- characteristics.
Steady State Operating Speed
Synchronous motor
Induction motor
Separately excited/ shunt DC motorSeries DC motor
SPEED
TORQUE
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Steady State Operating Speed• At constant
speed, Te= TL
• Steady state speed is at point of intersection between Te
and TL of the steady state torque characteristics
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TLTe
Steady state Speed, r
Torque
Speedr2r3
r1
By using power electronic converters, the motor characteristic can be varied
Steady State Stability Drives operate at steady-state speed (when Te = TL) only
if the speed is of stable equilibrium.
A disturbance in any part of drive causes system speed to depart from steady-state point.
Steady-state speed is of stable equilibrium if:
system will return to stable equilibrium speed when subjected to a disturbance
Steady-state stability evaluated using steady-state T-characteristic of motor and load.
Condition for stable equilibrium:
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m
e
m
L
d
dT
d
dT
(9)
Steady State Stability Evaluated using steady-state T- characteristic of
motor and load.
Assume a disturbance causes speed drop to r’
At the new speed r’,
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Te TL
Steady-state point A at speed = r
r
r’
Te’TL’
Te’ > TL’
motor accelerates
operation restored to steady-state point
m
TSteady-state speed is of
stable equilibrium
m
e
m
L
d
dT
d
dT
dt
dJTT m
Le
Steady State Stability Let’s look at a different condition!
Assume a disturbance causes speed drop to r’
At the new speed r’,
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TeTL
Steady-state point Bat speed = r
r
r’
TL’Te’
Te’ < TL’
motor decelerates
operation point moves away from steady-state point
m
TPoint B is at UNSTABLE equilibrium
m
e
m
L
d
dT
d
dT
dt
dJTT m
Le
Torque-Speed Quadrant of Operation
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m
Te
Te
m
Tem
Te
m
T
•Direction of positive (forward) speed is arbitrary chosen
•Direction of positive torque will produce positive (forward) speed
Quadrant 1Forward motoring
Quadrant 2Forward braking
Quadrant 3Reverse motoring
Quadrant 4Reverse braking
P = +ve
P = -ve
P = -ve
P = +ve
meTP
Electrical energy
Mechanical energy
MOTOR
P = + ve